The field of the invention relates to the measurement of visual sensitivity of contrast as a function of spatial frequency.
It is worth noting the contrast between testing techniques used to evaluate human sensory responses. In particular, note the disparity between testing of the visual and auditory senses. It is generally known that the standard test for the measurement of auditory acuity consists of presenting a sequence of audio signals, which vary in both amplitude and frequency, to acertain the threshold of audio sensitivity. An automated version of this testing technique was introduced by G. von Bekesy (ACTA OTO-LARYNGOL, 35:411 (1947)), and led to the various types of Bekesy audiometers or automatic audiometers to test hearing. On the other hand, conventional visual examinations singularly rely upon Snellen or similar type acuity charts, in near-total oblivion to the concurrent effects of contrast sensitivity and spatial frequency of visual acuity. Since the Snellen type acuity test consists of letters, typically L or E, set in a highly contrasted background, and these letters consist of luminance variations in terms of amplitude and spatial frequency over a very limited range, defined by the letter shape and size, it fails to adequately consider the functional relationship between contrast sensitivity and spatial frequency on visual acuity.
Advanced artisans have come to recognize the importance of the interrelationship between contrast sensitivity and spatial frequency in the visual process. Though the link is known, the difficulty in testing for these parameters in a simple, rapid, quantifiable manner has prevented widespread implementation.
An article by L. Ronchi et al, "Some Remarks on Opthalmic Test Types" in the American Journal of Optometry and Archieves of American Academy of Optometry, June, 1972, pp. 491-496 (copy in class 351/32) discusses some basic problems concerning visual test charts, such as size progression and target selection. He refers to an article by Kelly, "J.sub.o Stimulus Pattern for Visual Research" J. Opt Soc. Am., 50 (11)1115-1116, 1960 on a test pattern comprising concentric circles which can differ from one another in frequency and/or modulation depth.
A. P. Ginsburg has a pending patent application Ser. No. 282,997 filed July 14, 1981, now U.S. Pat. No. 4,365,873, for a "Spatial Frequency and Contrast Sensitivity Test Chart" and a corresponding report AFAMRL-TR-80-121 on "Proposed New Vision Standards for the 1980's and Beyond: Contrast Sensitivity". Each includes a figure showing examples of sine-wave gratings with low, medium, and high spatial frequencies at low and high contrast, and also includes definitions of spatial frequency and contrast. It is noted that the gratings will have different visibilities depending upon viewing distance due to the visual filtering characteristics of the observer. The Ginsburg patent application and report are hereby incorporated by reference.
R. F. Haines, U.S. Pat. No. 3,737,217 for "Visual Examination Apparatus", covers an automated apparatus for measuring visual sensitivity and mapping blind spot location. It includes a projection system for displaying to a patient a series of visual stimuli, a response switch enabling him to indicate his reaction to the stimuli, and a recording system (a two-pen plotter) responsive to both the visual stimuli per se and the patient's responses.